The present invention is associated with nonreciprocal circuit devices, such as isolators and circulators. Such nonreciprocal circuit devices are now required to have a smaller size. The ability to achieve a reduction in the size of the nonreciprocal circuit device depends on the ceramic substrate used therein. The ceramic substrate in the nonreciprocal circuit device may include a capacitor and a resistor. For example, when the device is used in a high frequency range of 800 MHz to 1.5 GHz, a capacitor is required to have a capacitance of about 100 pF. To achieve the reduction in size while maintaining the capacitance, the dielectric constant of the ceramic material constituting the ceramic substrate must be increased.
An example of a ceramic material composition that is used as a material for a ceramic substrate and that can achieve such a high dielectric constant is described in Japanese Unexamined Patent Application Publication No. 2002-97072 (Patent Document 1). Patent Document 1 proposes a ceramic material composition which can be sintered at as low as 1,000° C. or lower, can be co-fired with a metal such as Ag, and which can be used for forming a dielectric ceramic having a high dielectric constant, a high Q, and a small absolute value of the temperature coefficient of dielectric properties.
The ceramic material composition described in Patent Document 1 includes a BaO—TiO2—ReO3/2 ceramic composition (wherein Re represent a rare-earth element) as a filler; and a Li2O-MO—ZnO—Al2O3—B2O3—SiO2 glass composition (wherein M represents at least one element selected from Ba, Sr, Ca, and Mg).
The ceramic material composition described in Patent Document 1 preferably includes 65 to 85 percent by weight of the BaO—TiO2—ReO3/2 ceramic composition. Since the content of the BaO—TiO2—ReO3/2 ceramic composition is relatively high, sinterability tends to be degraded. To improve the sinterability, a low-viscosity glass needs to be used. Therefore, Li2O is incorporated in the glass composition to reduce the viscosity of the glass.
However, it has been found that when the ceramic material composition described in Patent Document 1 is used to produce a ceramic substrate included in the above-described nonreciprocal circuit device, the resistance characteristics of a resistor in the ceramic substrate disadvantageously decreases. From the results of experiments, the inventors found that the reason for the decrease in resistance characteristics was that a glass component in the ceramic material composition reacts with a resistive component in the resistor.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-97072